swift solution for the 8 queens problem (command line only)
// 12x12
// 11x11 Solutions Found: 14200 Positions checked: 10103868
// 10x10 Solutions Found: 2680 Positions checked: 1806706
// 9x9 Solutions Found: 724 Positions checked: 348150
// 8x8 Solutions Found: 92 Positions checked: 15720
// 7x7 Solutions Found: 40 Positions checked: 3584
// 6x6 Solutions Found: 4 Positions checked: 894
// 5x5 Solutions Found: 3 Positions checked: 220
// 4x4 Solutions Found: 2 Positions checked: 220
// 3x3 Solutions Found: 0 Positions checked: 18